I created a patch in Max/MSP that uses this principle (shepard tones or shepard pitch). The essence is that a bunch of tones that are octaves apart all rise or fall in pitch at the same speed. When rising, they fade out above a certain frequency, and at the same time tones fade in from a certain bottom frequency.

I did this by having lots of these tones that extend beyond the hearable range, and putting a band pass filter over everything to create the fading behaviour. Changing the frequency and width of the filter changes the frequency range of the shepard tone.

This may seem random but the actual inventor of 'Shepard Tones' was James Tenney. Tenney was a mentor of mine who passed away in my last semester at CalArts. At CalArts we liked to call this acoustic phenomena, "Tenney Tones" (has a nice ring to it). He was the first to use it in his song (For Ann (Rising)). He actually worked under Shepard at Bell Labs and never received credit for his work. In the scientific community it is common for the head of a lab to take credit for a discovery or important work even if they were not directly involved with it.

He never tried to push this idea on us, but I heard it from other people who worked with and around him during the 60's leading me to believe it is true. Just want him to have the credit he is due as he was a brilliant man and a fun person to be around.